Download mark scheme - A-Level Chemistry

Paddington Academy
AS-LEVEL PAPER 1 PP13 MS
1.
(a)
(i)
Macromolecular / giant covalent / giant molecular / giant atomic
If covalent, molecular, giant, lattice, hexagonal or blank mark
on.
If metallic, ionic or IMF chemical error CE = 0 for (a)(i), (a)(ii)
and (a)(iii).
(ii)
Delocalised electrons / free electrons
1
1
Able to move / flow (through the crystal)
Allow M2 for electrons can move / flow.
Ignore electrons can carry a current / charge.
1
(iii)
Covalent bonds
1
Many /strong / hard to break / need a lot of energy to break
M2 dependent on M1.
Ignore van der Waals’ forces.
1
(b)
(i)
(Giant) metallic / metal (lattice)
If FCC or BCC or HCP or giant or lattice, mark on.
If incorrect (b)(i), chemical error CE for (b)(ii) and (c)(ii).
(ii)
Nucleus / protons / positive ions and delocalised electrons (are attracted)
QWC Must be delocalised electrons – not just electrons.
Chemical error = 0/2 for (b)(ii) if other types of bonding or
IMF mentioned.
1
1
Strong attraction
Allow strong metallic bonding for one mark if M1 and M2 are
not awarded.
1
(c)
(i)
Layers of atoms/ions slide (over one another)
Do not allow just layers.
(ii)
(Strong) (metallic) bonding re-formed / same (metallic) bonding / retains
same (crystal) structure / same bond strength / same attraction between
protons and delocalised electrons as before being hammered or words to
that effect
If IMF, molecules, chemical error CE = 0/1 for (c)(ii).
If metallic not mentioned in (b)(i) or (b)(ii) it must be
mentioned here in (c)(ii) to gain this mark.
Do not allow metallic bonds broken alone.
Ignore same shape or same strength.
1
1
(d)
(giant) Ionic
If not ionic, chemical error CE = 0/3
1
Between + and – ions / oppositely charged ions or Mg2+ and O2–
If molecules mentioned in explanation lose M2 and M3
Allow one mark for a strong attraction between incorrect
charges on the ions.
1
Strong attraction
1
[13]
Page 1
Paddington Academy
2.
(a)
(i)
Mol SO3 = 5.2
1
Mol SO2 = 2.8
1
(ii)
Penalise expression containing numbers or V
Allow ( ) but must have all brackets. If brackets missing but
otherwise correct, penalise here but mark on
Ignore subsequent correct working
If Kc wrong (wrong powers or upside down etc) can only
score M1 in (a)(iv)
1
(iii)
mol dm–3
Allow conseq to their wrong Kc
1
(iv) If Kc wrong in (a)(iv) (wrong powers or upside down etc) can only score M1
Values from (a)(i)
or
Alternative values
1
M1 For dividing all three by volume – if volume missed or used
wrongly,
lose M1 & M2 but can score M3 conseq
M2 insertion of values (allow conseq use of their wrong values from (a)(i))
AE (–1) for copying numbers wrongly or swapping two
numbers
1
Values from (a)(i)
M3 = 0.0338 or 0.034
(allow 0.03376 to 0.035)
Min 2 sfs
Ignore units in (a)(iv)
If vol missed score only M3
Values from (a)(i)
0.406 - allow values between 0.40 (if correctly rounded) and
0.41
Alternative values
M3 0.0153 or 0.015
(allow 0.015 to 0.017)
Min 2 sfs
Ignore units in (a)(iv)
from alternative values allow 0.18 to 0.184
1
(b)
(i)
Increase or more moles (of oxygen) or higher
(ii)
No change or no effect or none or (remains) same
1
1
Page 2
Paddington Academy
(c)
M1
T1
If T2 CE = 0
1
M2
(At Temp,T2, when Kc is lower) Equm/reaction moves to left or
towards reagent or towards SO3 OR moles SO3 increases
M3
This reverse reaction is exothermic,
OR
(forward) reaction is endothermic
if Temp is increased Equm/reaction moves to right or towards product
or towards SO2 OR moles SO2 increases
OR
(forward) reaction is endothermic
if Temp is decreased Equm/reaction moves to left or towards reagent
or towards SO3 OR moles SO3 increases
1
M3
M2
M3
M2
1
[12]
3.
Hydroxide:
solubility increases
1
Sulphate:
solubility decreases
[BOTH inc/dec allow 1/2]
[Allow correct solubilities of top (Mg) and bottom (Ba) cpds]
1
Add:
BaCl2(aq) / Ba(NO3)2(aq) / Ba(OH)2(aq)
[Not solid added]
[Not Ba2+ / Ba / Ba + HCl / Pb(NO3)2(aq)]
[If BaSO4 / H2SO4 used, M3 to M6 = CE = 0]
[Allow any sensible nitrate test as an alternative to
the sulphate test]
1
[Note: If M3 not awarded but test would work, allow correct observations
and equations]
Na2SO4
white precipitate / solid / suspension
[not cloudy/milky]
1
NaNO3
no change
1
BaCl2 + Na2SO4 → BaSO4 + 2NaCl
Accept ionic equation
1
[6]
4.
(a)
gains electrons (1)
or accepts/takes electrons
Allow an electron
or just ‘gains’
or reduction is gain of electrons, but NOT OILRIG even if
stated
Do not allow mention of electron pair(s)
(b)
(i)
1
(ii)
Oxidising agent: Ag+ (1) (or Ag I)
Reducing agent: SO2 (1) (or SVI, not sulphur)
SO2 + 2H2O → SO42– + 4H+ + 2e– (1) (or H2SO4 + 2H+ + 2e–)
allow e i.e. no charge
penalise E– once only
allow –e– on LHS
3
Page 3
Paddington Academy
Fe2+ → Fe3+ + e– (1)
5 (1) (or V or +5)
ClO3– + 6H+ + 6e– → Cl– + 3H2O (1)
ClO3– + 6H+ + 6Fe2+ → Cl– + 3H2O +6Fe3+ (1)
Mark parts (i) to (iv) independently
(c)
(i)
(ii)
(iii)
(iv)
(d)
Equation: Mg + S → MgS (1)
allow FeS + Mg → MgS + Fe
allow Ca
Oxidising agent: S (1)
Only award mark if first answer given unless no first answer
then can allow
4
2
[10]
5.
Mark
Range
4-5
2-3
0-1
(a)
The marking scheme for this part of the question includes an overall
assessment for the Quality of Written Communication (QWC). There
are no discrete marks for the assessment of QWC but the
candidates’ QWC in this answer will be one of the criteria used to
assign a level and award the marks for this part of the question
Descriptor
an answer will be expected to meet most of the criteria in the level
descriptor
– claims supported by an appropriate range of evidence
– good use of information or ideas about chemistry, going beyond
those given in the question
– argument well structured with minimal repetition or irrelevant
points
– accurate and clear expression of ideas with only minor errors of
grammar, punctuation and spelling
– claims partially supported by evidence
– good use of information or ideas about chemistry given in the
question but limited beyond this
– the argument shows some attempt at structure
– the ideas are expressed with reasonable clarity but with a few
errors of grammar, punctuation and spelling
– valid points but not clearly linked to an argument structure
– limited use of information or ideas about chemistry
– unstructured
– errors in spelling, punctuation and grammar or lack of fluency
Kills bacteria / prevents bacterial diseases QWC
1
Chlorine is a toxic substance
1
Cl2 + H2O → HCl + HClO
1
Page 4
Paddington Academy
(b)
Cl2(aq) to Br–(aq);
yellow-orange or yellow-red or QWC
yellow-brown solution
1
2Br– + Cl2 → 2Cl– + Br2
or molecular equation
1
Cl2(aq) to I–(aq);
brown/black solution formed or QWC
black/brown/grey ppt/solid
1
2I– + Cl2 → 2Cl– + I2
or molecular equation
1
(c)
Bromide:
Brown/orange fumes
1
Bromine produced
1
Sulphur dioxide produced
1
Iodide:
Purple fumes or black/brown/grey solid QWC
or smell of bad eggs
1
Iodine produced
1
SO2, S, H2S produced (one mark each)
3
Half-equations
2Br– → Br2 + 2e–1
OR 2I– →I2 + 2e–
1
H2SO4 + 2e– + 2H+ → SO2 + 4H2O
OR H2SO4 + 6e– + 6H+ → S + 4H2O
OR H2SO4 + 8e– + 8H+ → H2S + 4H2O
1
Overall equation Any correct equation based on half-equations QWC
1
[18]
6.
(a)
Ammonia is an alkali / would react with HCl
Do not allow ‘fair test’ or ‘to improve accuracy’ without further
qualification
1
(b)
Batches may vary / batch might be contaminated
Do not allow ‘fair test’, ‘to improve accuracy’, ‘reliable’ or
‘reproducible’ without further qualification
(c)
NH4NO3 → N2O + 2H2O
Accept multiples
(d)
(i)
1
1
ammonium chloride
134 / 26.2 = 5.11 £ per %
ammonium nitrate
175 / 35 = 5.00 £ per %
ammonium sulphate
111 / 21.2 = 5.24 £ per %
Must have some evidence of working to score this mark
Accept calculations of nitrogen content per pound
1
ammonium nitrate
Accept name or correct formula
Allow consequential answer from candidate’s results
1
(ii)
Washed / dissolved / leached from soil by rainwater / eutrophication
Allow root damage due to temperature drop when salt
dissolves
1
Page 5
Paddington Academy
(e)
17.6
Ignore precision of answer
Ignore units
Do not allow 18
1
[7]
7.
(a)
(CO2 from) burning (fossil) fuels
(b)
NaCl + CO2 + NH3 + H2O → NaHCO3 + NH4Cl
Allow multiples, including fractions.
Ignore state symbols.
(c)
CaO + 2NH4Cl → CaCl2 + 2NH3 + H2O
Allow multiples, including fractions.
Allow ionic equations.
Do not allow equations involving NH4OH or NH4+ on the right
hand side.
Ignore state symbols.
(d)
(i)
1
1
1
= (106) × 100 / (117 + 100(.1))
Do not penalise precision but must be to minimum of two
significant figures.
1
= 48.8
This answer without working scores 1 mark only.
1
(ii)
The percentage atom economy cannot be improved
OR
Sell the by-product / CaCl2 (solution)
Do not accept answers which refer to improving the
efficiency of the process.
1
(e)
It is used up but then regenerated later in the cycle / No overall consumption
of NH3
Allow ‘can act as a catalyst’.
1
[7]
8.
(a)
(i)
Mass loss would be too large / water would be lost when heating (so mass
incorrect)
Do not allow ‘to improve accuracy’ without qualification.
Do not allow ‘water is a product of the reaction’.
Do not allow ‘mass of crucible incorrect / too high’.
1
(ii)
Prevents loss of solid / potassium carbonate / potassium
hydrogencarbonate (from the crucible)
Do not allow ‘to improve accuracy’ without qualification.
Do not allow ‘stops anything escaping’.
(iii)
Errors in weighing are too high / percentage errors in (obtaining) the
mass are too high
Do not allow ‘hard to / can’t weigh very small amounts’
without further qualification.
1
1
Page 6
Paddington Academy
(b)
(i)
Mr of KHCO3 is 100(.1)
Do not penalise precision.
1
moles K2CO3 = 1 / (Mr × 2) = 0.005
If factor of 2 missing can only score first mark (M1).
Allow consequential answer on incorrect Mr of KHCO3
1
mass of K2CO3 = 0.005 × 138.2 = 0.69(0)
Correct mass without working scores one mark only (M3).
1
(ii)
Reaction / decomposition incomplete
Do not allow ‘a wet crucible was used’.
Ignore references to impurity and / or experimental errors.
1
[7]
Page 7